1. Field of the Invention
The present invention relates to a process for production of sulfonium compounds and novel methylthiophenol derivatives. More particularly, it is concerned with a process for producing sulfonium compounds, dialkylsulfoniophenol derivatives represented by the general formula (III), by sulfoniumating alkylthiophenol derivatives represented by the general formula (I) with dialkyl sulfate represented by the general formula (II). ##STR2##
In the general formulas (I), (II) and (III), R.sup.1 and R.sup.2 may be identical or different and are each a lower alkyl group having 1 to 4 carbon atoms, and X is a hydrogen atom, a halogen atom, or a lower alkyl group having 1 to 4 carbon atoms.
In the general formulas (I) and (III), R is a tertbutyloxy group, a tert-amyloxy group, a p-methoxybenzyloxy group, a 2-(trimethylsilyl)ethoxy group, a 1-adamantyloxy group, a bornyloxy group, or an isobornyloxy group.
Sulfonium compounds represented by the general formula (III) are useful compounds as reagents for introduction of an acyl group as a protecting group to various compounds in the organic chemical field, e.g., synthesis of peptides, because they exhibit acylating action in an aqueous solution.
The term "acyl" as used herein refers to an atomic group derived by removal of a hydroxy group from a carbonic acid monoester.
The present invention further relates to novel methylthiophenol derivatives capable of being precursors for the above sulfonium compounds and more specifically to novel methylthiophenol derivatives represented by the general formula (I'): ##STR3## wherein R is the same as defined above.
2. Description of the Related Arts
For production of the sulfonium compounds represented by the general formula (III), a method of reacting acid halides, i.e., carbonylhalogenide compounds with p-dialkylsulfoniophenol alkylsulfate in the presence of a base has been generally employed.
Japanese Patent Application Laid-Open No. 8365/1988, for example, discloses a method in which acid chloride is used as the acid halide, and triethylamine is used as the base.
This method, however, is not necessarily satisfactory for practical use, because a complicated operation is required for separation and purification of the objective products, sulfonium compounds of the general formula (III), from by-products of triethylamine hydrochloride.
Moreover, in this method, the desired sulfonium compound cannot be obtained unless an unstable compound such as tert-butoxycarbonyl chloride or p-methoxybenzyloxycarbonyl chloride is used as the acid halide. In some cases, the desired product cannot be obtained at all. Thus the method is not desirable from an economic standpoint and is unsuitable for pracical use.
A method of converting thioanisole into the corresponding sulfonium with dimethyl sulfate is known (H. M. Gilow & G. L. Walker, J. Org. Chem., 32, 2580 (1967)).
When a p-methylthiophenol derivative, one of the alkylthiophenol derivatives of the general formula (I), is intended to be converted into the corresponding sulfonium compound with dimethyl sulfate, besides the desired sulfonium compound, a large amount of decomposition product, p-dimethylsulfoniophenol methylsulfate, is produced. Thus the yield of the desired sulfonium compound is low, and in some cases, the desired product is not obtained at all but only decomposition product is formed.
The present invention is intended to provide an industrially advatageous process for producing the desired sulfonium compound with high efficiency and at low production costs by reacting alkylthiophenol derivatives and dialkyl sulfate.
The present inventors made investigations on a method of synthesis of sulfonium compounds represented by the general formula (III) in order to overcome the above problems. As a result, it has been found that in the reaction of alkylthiophenol derivatives represented by the general formula (I) with dialkyl sulfate represented by the general formula (II), if the reaction is carried out in the presence of specified inorganic compounds, the decomposition reaction of the sulfonium compounds can be inhibited and the desired products can be obtained with high efficiency.
Moreover, in the course of the above investigations, it has been found that among the alkylthiophenol derivatives of the general formula (I) to be used as the starting material for the above reaction, specified compounds are novel compounds. Based on the findings, the present invention has been accomplished.